1. Technical Field
The present invention relates to an output driver and, more particularly, to an output driver having improved operation characteristics.
2. Description of the Related Art
Inter-Symbol Interference (ISI) is noise occurring when an output driver drives a large load (for example, a loading capacitor), and mainly occurs when a non-constant data signal pattern is output. In particular, it is a phenomenon in which the waveform of an output data signal is distorted in a channel because a currently output data signal is output before a previously output data signal has been sufficiently stabilized in the channel when the rising time and falling time of an output data signal are longer than the period of a clock signal. That is, the waveform of a current output data signal varies depending on the pattern of a previous output data signal.
FIG. 1 is a diagram illustrating the waveform of an output data signal attributable to ISI.
Referring to FIG. 1, an output data signal DOUT swings between a voltage level VOL and a voltage level VOH in an interval (interval Δt1) in which an input data signal DIN continuously toggles between a low level and a high level. In this case, when there is a large load (a loading capacitor and a resistor) in an output node and operation speed is high, the swing of an output data signal may be restricted. However, the level of the output data signal DOUT continuously increases above the voltage level VOH by ΔV in an interval (Δt2) in which the input data signal DIN is continuously at a low level. Therefore, the output data signal DOUT swings between a voltage level VOL+ΔV and a voltage level VOH+ΔV in an interval (Δt3) in which the input data signal DIN continuously toggles between the low level and the high level. As described above, the output data signal DOUT has different waveforms in the intervals Δt1 and Δt3, so that data signal pattern-dependent skew occurs. As a result, a timing margin and a voltage margin decrease in a receiver receiving the output data signal DOUT.
In FIG. 1, reference character VH designates the maximum output voltage of the output driver, reference character VL designates the minimum output voltage thereof, and reference character VTH designates a logic threshold voltage.
In order to overcome the problems, an output driver, which performs a pre-emphasis operation, has been conventionally used. The output driver includes a main driver for receiving a currently input data signal, a pre-emphasis driver connected in parallel to the main driver and configured to receive a previously input data signal, and a storage block for storing the previously input data signal. As a result, the conventional output driver further includes the pre-emphasis driver and the storage block, so that the size thereof is large and current for operating the pre-emphasis driver and the storage block is additionally required, thereby increasing power consumption.